1. Field of the Invention
The present invention relates to a superconducting cable and a superconducting cable line including the superconducting cable. More particularly, the invention relates to a superconducting cable in which damage caused by a failure, for example, a short-circuit failure, can be reduced, and also relates to a superconducting cable line using this type of superconducting cable.
2. Description of the Related Art
Superconducting cables having a superconducting cable conductor formed of, for example, Bi-based high critical temperature (Tc) superconducting tapes, are known (see, for example, Japanese Unexamined Patent Application Publication No. 2001-325836). FIG. 3 is a sectional view illustrating a three-core three-phase superconducting cable 100 incorporating a plurality of cable cores 102. In this superconducting cable 100, the three cable cores 102 are stranded and housed in a thermal insulation pipe 101.
The thermal insulation pipe 101 is a double pipe structure such that a thermal insulating material (not shown) is disposed in the space between an outer pipe 101a and an inner pipe 101b, and the space is kept in a vacuum state. Each cable core 102 includes, sequentially from the center, a former 200, a superconducting conductor layer 201, (which is made of with superconducting wires, for example, and hereinafter referred to as the “superconducting conductor layer”), an electrical insulating layer 202, a shielding layer 203, and a protective layer 204. The superconducting conductor layer 201 and the shielding layer 203 are both superconducting layers formed by spirally winding superconducting wires in a multilayer form around the former 200 and the electrical insulating layer 202, respectively. Normally, a space 103 between the inner pipe 101b and each cable core 102 serves as a coolant channel. An anticorrosion layer 104 made of polyvinyl chloride is provided at the outer periphery of the thermal insulation pipe 101.
When the above-described multiphase superconducting cables are connected to each other or such a cable is connected to a normal conducting cable, or when a termination structure is formed, such work is performed for each cable phase, that is, by splitting the cable cores. In such a case, the cable cores are split in a splitter which is maintained at a cryogenic temperature by a coolant, and the cable cores are held in the splitter such that they are separated from each other.
When a short-circuit failure occurs in a normal conducting cable, a large current flows in the conductor. In a 66 kV normal conducting cable, for example, about 31.5 kA short-circuit current flows in the conductor. As in the normal conducting cable, a large short-circuit current flows in a superconducting conductor layer in case of a short-circuit failure.
Under normal operation, since the current flowing in a superconducting cable is a few kilo-amperes, the direct current (DC) critical current value Ic of the superconducting layer is set to be, for example, Ic≧√2×Io with respect to the alternating current (AC) rated current Io. However, since a superconducting conductor forming the superconducting layer generally is expensive, it is difficult in view of cost-effectiveness to increase the margin with respect to the critical current value Ic. When a short-circuit failure occurs to cause a short-circuit current to flow in the superconducting conductor layer having a relatively small margin in terms of the critical current value Ic, the superconducting state cannot be maintained in excess of the critical current value Ic, causing the superconducting layer to transit from a superconducting state to a normal conducting state. That is, so-called “quenching” occurs to cause the superconducting layer to generate heat, and the superconducting cable may be damaged due to an increase in the temperature.
In a multiphase superconducting cable, such as that shown in FIG. 3, when the cable cores housed in a splitter generate heat, the increased temperature vaporizes a coolant such as liquid nitrogen around the cable cores, which may result in excessive increase of the pressure in the splitter, consequently damaging the cable.
Such damage can be caused in any portion of the superconducting cable, and thus, a damaged portion is difficult to determine.